The present invention relates to a vehicle steering device which steers the steered wheels by operating a steering member.
Recently, there has been provided a vehicle steering device based on a so-called steer-by-wire system (referred to also as SBW) in which a mechanical connection between a steering member, such as a steering wheel, and the steered wheels is removed, and a part of a steering force transmitting system is formed by an electrical path.
In a device disclosed in JP-A-01-233170, a steering position of a steering wheel is detected by an encoder provided on a steering shaft, and an advancing direction-of a vehicle is detected by use of a yaw rate gyro. The steered wheels are controlled in steering so that a difference between an advancing direction change instructing value based on a detected steering position and an actual advancing direction change quantity based on the detecting result by the yaw rate gyro, becomes zero.
In the steer-by-wire system of this type, however, a fail-safe measure for trouble, for example, wire disconnection, when it occurs in an electric motor as an actuator for the steering, is of importance.